1. Field of the Invention
This invention relates to microstrip transmission lines operating in the millimeter wave region of the frequency spectrum and more particularly to a switchable microstrip Y-junction circulator for use with such microstrip transmission lines.
2. Description of the Prior Art
A problem arising in millimeter wave frequency applications which utilize microstrip transmission line and other planar circuit components is the switching of the millimeter wave frequency signals involved. Because of the extremely small physical size of the microstrip and other components used in this region of the frequency spectrum, the switching function should be accomplished by a device of small physical size which preferably consumes very little energy to accomplish the switching function. Although active diode elements are often used to perform the switching function, they require a constant consumption of power which is undesirable for many applications. Furthermore, the design of the switching device should be as simple as possible to facilitate manufacturing of the device and to keep the manufacturing costs as low as possible. Since switches are often called upon to perform modulation functions, a suitable switching device for applications of this type should also be capable of functioning as a modulator.
Y-junction circulators are non-reciprocal coupling devices having three ports which provide signal transmission from one port to an adjacent port while decoupling the signal from the remaining port. They are used in radar system front ends as duplexers to couple the transmitter and receiver to the single radar antenna. They are also used in many other applications such as signal generator protection circuits and transmitter injection locking circuits, for example. A circulator could be used to perform a switching function if the circulator itself could be made switchable. For a circulator to be switchable, the rotational direction of circulator coupling action must be easily reversed so that a signal applied to one port of a Y-junction circulator may be switched to either of the two adjacent ports. With the great increase in use of planar circuitry using microstrip transmission lines in millimeter wave frequency applications because of the resulting reduction in size and weight of the equipment involved, a need has arisen for a Y-junction circulator which is suitable for use with such planar circuitry and microstrip transmission lines and which is easily switchable.
Conventional millimeter wave microstrip circulator designs generally utilize a small ferrite disc or "puck" which has metallized ends and which is disposed in a hole in the microstrip transmission line substrate at the point where the microstrip lines to be coupled meet. The puck has a thickness which is equal to the thickness of the microstrip transmission line substrate so that the metallized ends of the puck may be electrically connected to the microstrip conductors and the metal ground plane of the transmission line. When a unidirectional magnetic field is applied between the ends of the puck, a clockwise or counterclockwise non-reciprocal coupling action is produced between the microstrip lines which are joined at the puck. The clockwise or counterclockwise coupling direction may be reversed by reversing the direction of the applied magnetic field. A circulator of this type is shown and described in U.S. Pat. No. 3,456,213 issued July 15, 1969. This circulator is not easily switchable, however, with the permanent magnet arrangement disclosed by the patentee. If the magnetic field is created by a helically-wound coil, the coil would require a pole structure which would increase the size and weight of the circulator to the point where its use would no longer be feasible in millimeter wave applications.
The manufacturing and assembly costs of the puck-type circulators are relatively high because the ferrite puck must be fitted into the substrate hole with a very close tolerance fit to minimize line impedance variations and to reduce insertion losses. Additionally, if the dielectric constant of the microstrip substrate is different from the dielectric constant of the ferrite, a matching transformer configuration is required which further increases the aforementioned costs. Furthermore, the ferrite puck arrangement is not readily adapted to the monolithic design and automated assembly techniques which must be utilized in the fabrication of microstrip circuits in order to reduce their complexity and cost.